Arch dam at Seathwaite Tarn

 

Seathwaite Tarn was turned into a reservoir for Barrow over a hundred years ago. The dam is actually curved - seen behind the spillway in the photo above. Most dams I've seen are straight. This one has its curve facing the water. It occurred to me that an arch is a strong shape because the weight of stone above an arched window pushes the arch blocks into each other, locking them. Here, it is the water pressure that does that job. Turns out this type of dam has a name: an arch dam. It is neatly summarised by The British Dam Society who say that this type is rare in the UK because it needs strong rock foundations, presumably to be able to push back on the blocks that make up the dam. That is the case here.